Technically, a yawl has the mizzen mast stepped aft of the rudder post. I spent some years on a yawl, and most of the folks I knew with that rig dropped the mizzen if the apparent wind was much forward of the beam. The only reason for the rig was "rule beating" under the old CCA race rules.
As far as as true ketches go, Jeff knows more than I do, and what I do know agrees with his post.